As one measuring the oil content, a technology described in Patent Document 1 is known. In Patent Document 1, a quartz crystal microbalance (hereinafter referred to as QCM) is used, droplets of water soluble ink which is a measuring object are dropped onto a quartz crystal oscillator by an ink jet, by the change amount of the resonance frequency (fundamental frequency) of the quartz crystal oscillator before dropping the droplets and the resonance frequency measured after dropping the droplets, the mass of the deposit that is the water soluble ink on the quartz crystal oscillator is measured. In other words, using the characteristics of the change of the resonance frequency (called also as natural frequency) and the deposit amount on the surface thereof, the oil content in the sample water of a minute amount is measured.